Ionization radiation, as from cobalt-60 sources, has become widely used for such diverse purposes as the modification of certain polymers, the sterilization of a variety of packaging materials and medical supplies, the pasteurization of foodstuffs and cosmetics, and other applications, all involving the irradiation of discrete packages, many of which are traditionally rectangular in cross section. In order to make efficient use of the radiation energy, which is expensive, it is desirable to pack the products to be processed as closely as possible around the source. Furthermore, within any such array, the exposure would be nonuniform, and in order to obtain the necessary uniformity of exposure, it is desirable that each package of product occupy each position in which the exposure rate is significantly different, for approximately the same length of time as every other package of product. Finally, in order to minimize labor and maximize the use of the radiation, the shield and the equipment, it is also desirable to move product through the radiation field remotely.
By prior art means, numerous methods have been established for moving the product through a radiation field in carriers in such a manner that the product is close packed in a direction parallel to the plane of the source and irradiated first on one side, and then on the other to achieve a satisfactory degree of uniformity. Such prior art means also provide for close packing of product in the vertical dimension in said carriers. However, such prior art means do not provide for efficient packing in the dimension normal to the plane of the source. If products of highly uniform densities and dimensions are to be processed, prior art methods approach ideal close packing because the rows of product may be placed in close proximity to each other and still have sufficient clearance between rows to provide for reliable operation. However, for the irradiation of packages of varying thickness, prior art systems are seriously deficient in the use of available space; and for the processing of materials of varying densities, prior art systems frequently are unable to efficiently deliver the radiation at a desirable maximum-to-minimum dose ratio.
The purpose of the present invention is to provide a means whereby it is practical to move the product through a radiation field wherein the product to be irradiated is closely packed in the dimension normal to the plane of the source as well as in the vertical planes and in the dimension parallel to the plane of the source for a wide variety of package thicknesses. In this way, it is practical to achieve a modest increase in the efficiency of irradiating packages of uniform dimensions and a major improvement in the efficiency of processing product packages of different thicknesses Same major improvement permits a greater degree of freedom in the selection of package thickness to accommodate materials of different density thereby making it practical to achieve the desired degree of uniformity without sacrificing radiation or irradiator utilization efficiency.
The objectives of the present invention are accomplished by organizing product carriers in an irradiator in a novel way so that they traverse the source in a rank of two or more carriers instead of single file as in the case of prior art methods. Thus, the clearance between files that is required by prior art techniques is eliminated; and more important, the present invention permits the efficient use of a variety of carrier widths within the same irradiator. Consider, for example, a prior art four pass irradiator designed to accommodate cartons 18" thick. By prior art, carriers of perhaps 19" width would traverse the irradiation cell parallel to the source array in single file on tracks no closer than perhaps 20" apart. Assuming the source array is in an east-west plane, the product carriers would first proceed east on the outside track south of the source the length of the cell, then move north one file, proceed west the length of the cell, then be moved north to the inner track on the north side of the source, proceed east the length of the cell, and then north to the outer file, and west again to complete the fourth pass. Such prior art irradiator could not accommodate a package of greater thickness than 20" under any circumstances, and thinner packages are irradiated only at great sacrifice in system efficiency. A ten inch thick package, for example, would occupy only 55% of the space on the carrier, even if it were the ideal length and height, and if placed on 11" carriers, such packages would still traverse the irradiator on 20" centers resulting in a substantial loss in irradiation efficiency.
By practice of the present invention, however, the irradiator carriers are organized in a rank of two or more carriers on a rack which permits the inner carrier to be pushed as close to the source as reasonable clearance allows, and for adjacent carriers to be in contact with each other on each rack. A rack 48" wide for example, which would accommodate two or three 16" carriers, could also accommodate one 12" carrier, one 16" carrier and one 20" carrier, or any other combination up to the overall limit of 48" aggregate width while having the product as close to the source as possible in every case.
Under prior art practices, the ideal of packing product close to the source has been practical only by means of manual rearrangement or strict limitations on package size because prior art had not contemplated a practical arrangement where product could be moved remotely and automatically through an irradiation field in such a manner as to have each product carrier close packed both parallel and normal to the plane of the source, regardles of carrier width, over a wide range of carrier widths. The present invention overcomes this deficiency in principle by conceiving the hitherto untried method whereby carriers traverse the irradiation chamber parallel to the source array in a rank, rather than single file, and further providing means for packing the individual carriers in close proximity to each other within each rank and to the source. This ideal array is made practical by the introduction of a novel means of relocating the carriers on a moving rack after each circumnavigation of the source so that each carrier in turn occupies each position.